Algae are an extremely diverse group of aquatic plants. They range in size from single cells around a micrometre (1/1000 millimetre) in diameter, to seaweeds the size of trees. Like other plants, they use the energy of sunlight to convert carbon dioxide and water into sugars and oxygen.

There are currently around 700 recorded species of algae in Antarctica. They have been collected from as far south as 86° 29'.

In the waters and sea ice

The most important algae are the marine phytoplankton. These microscopic floating single cells have a huge diversity of size, shape and lifestyle. There are around 350 different species identified from Antarctic waters. Phytoplankton are the base of the food web on which all other marine organisms depend.

Phytoplankton play a significant role in influencing global climate. The Southern Ocean absorbs atmospheric carbon dioxide, the principal greenhouse gas. This is largely due to the uptake of carbon dioxide by phytoplankton. Some also produce compounds which, when released into the atmosphere, promote the formation of clouds. Clouds are important in climate control because they reflect much of the Sun’s energy back into space.

The sea ice around Antarctica often has rich growths of algae, often on the underside. Some 700 seaweeds have been reported from the Southern Ocean. 35% of these are not found anywhere else. In shallow waters, the sea floor is also covered with single-celled algae. Many bottom-dwelling animals rely on the algae as a food source.

On the land

On the Antarctic continent and sub-Antarctic islands, algae live in lakes and streams, on moist soil and in snow banks. On the continent, some algae also live in porous sandstone rocks and underneath translucent quartz rocks. In these places, moisture and light are freely available for their growth. Soil algae and their associated bacteria are important to the ecology of these areas. They contribute to the organic material, and help bind the soil particles together with the mucilage and slime they secrete.

Microscopic algae are also found in snow and ice on land in the coastal regions. In summer, the algae accumulate enough to colour the snow banks red, green, orange and grey. Snow algae are generally single-celled organisms, although some multicellular and filamentous forms exist.

The algae that constitute the photosynthetic part of lichens are often overlooked. Lichens thrive in extreme environments where many other plants do not. As a result, there is much lichen found growing across Antarctica. There are only a few species of lichen algae, but they are fundamental to many types of lichen.

Adapting to the environment

Many different environmental variables affect the types and amounts of Antarctic algae. Wind–driven mixing and iron availability are major controls in the sea. Nutrients and availability of water are important in non-marine habitats.

Increased UVB radiation from the ozone hole has very different effects on different algae.

Algae use several mechanisms to protect themselves from UV exposure. These include avoidance, protective pigments and repair mechanisms. If the UVB radiation continues to increase, it is likely that the amounts and ratios of different algae will change.